1. Technical Field
The present invention relates to a method and apparatus for properly orienting a modular downhole tool such as a perforation gun. Specifically, it relates to an apparatus that allows the tool to be positioned with a particular angular orientation.
2. Description of the Related Art
Perforating casing in a wellbore involves lowering a perforating gun or guns into the wellbore, positioning the gun(s) adjacent to the geological zone intended to be perforated, and detonating the perforating gun(s). The geological zone of interest is generally laid down in a horizontal plane. However, geological formation attributes such as porosity and/or permeability may vary around the circumference of the wellbore. A preferred vertical plane may exist within the horizontal plane of the formation for porosity and/or permeability. In such a preferred vertical plane, the porosity and/or permeability of the formation may be more advantageous for producing hydrocarbons from the formation. Therefore, orienting the perforating gun(s) with respect to the preferred vertical plane of the formation would tend to increase the producibility of the well. Other advantages gained from orienting the perforating gun(s) with respect to the direction of the preferred vertical plane might be realized in mechanical fracturing operations, as well as in acidizing operations performed on the formation of interest. By orienting the perforations with the preferred vertical plane, the hydraulic pressure for fracturing and the reactive acid from acidizing are aligned with the best possible porosity and/or permeability vertical plane, which allows each to penetrate deeper into the formation. Also, in a situation where a well is blowing out and a relief well is drilled for diverting the blowout pressure and flow, complete control over the relief well is maintained by perforating into the blowout well from the relief, rather than drilling into the blowout. Oriented perforating provides a means for maximizing the communication between wells.
A problem with current technology is the inability to orient slickline perforating guns along the maximum horizontal stress while simultaneously perforating an entire zone in a given vertical direction in order to lessen the effects of sanding in a newly perforated well.
The perforating gun(s) can be conveyed into the well using either tubing or wireline as a conveyance. By using tubing as the conveyance, the perforating charges within the perforating gun may be oriented at a 180.degree. phase from each other, which enables the well operator to perforate only in the angular orientation of the preferred vertical plane without wasting perforating charges oriented at other directions. This is performed by lowering the perforating gun(s) into the wellbore attached to the tubing, and once the perforating gun is positioned across the formation of interest, a direction survey is performed for finding the orientation of the perforation gun(s) with respect to the preferred vertical plane. Running the perforating gun(s) on tubing requires a special anchoring device on bottom of the stringer or, without anchoring device, firing is limited to a mechanically operated firing head. With an anchoring device the perforation gun(s) can be fired with pressure.
A wireline or slickline unit is normally used for running the direction survey. By using a wireline the operator gets a real time readout of the orientation of the perforating gun at the surface. Once the orientation of the perforating gun(s) is determined, the tubing is rotated a corresponding amount for angular orientation of the perforating gun(s) with respect to the preferred vertical plane. Normally, the wellbore is then perforated after rotating the tubing; however, in the case of highly deviated and/or exceptionally deep wells, a second directional survey might be performed in order to confirm that the rotation applied to the tubing at the surface has been completely transferred to the downhole perforating gun(s). While use of tubing as the conveyance for the perforating gun(s), in combination with performing a direction survey, guarantees orientation accuracy, the procedure is relatively expensive, as it requires the use of both a workover rig for conveying the tubing, and, a wireline or slickline unit for running the direction survey.
Alternatively, the perforating gun(s) can be conveyed into the well using wireline as a conveyance, thus eliminating the need for a workover rig. One method of perforating using wireline is by employing the modular perforating method and apparatus disclosed in U.S. Pat. No. 5,366,014, titled "Method And Apparatus For Perforating A Well Using A Modular Perforating Gun System," by George, incorporated in its entirety here within by reference. FIGS. 1A and 1B depict perforating a wellbore using a modular gun assembly. Initially, gun hanger 110 is inserted into casing 100 and positioned below formation 120 using collar locator 140, which is attached through a cable head to a cable, either wireline or slickline. Gun hanger 110 is then set using a running tool 150 (shown in a cutaway view in order to view stinger 130B). Running tool 150 and collar locator 140 are then retrieved from the wellbore, leaving stinger 130A in an upturned position on top of gun hanger 110.
With gun hanger 110 in position, modular perforating gun 160, which includes upper gun head 180 and alignment skirt 170, is run inside casing 100 using collar locator 140 and running tool 150, similar to running gun hanger 110 into the wellbore. Alignment skirt 170 is coupled to upturned stinger 130A, which causes perforating charges 162 to be positioned at depths adjacent to formation 120 (FIG. 1A depicts perforating gun 160 just prior to coupling of skirt 170 and stinger 130A). Also included are centralizers 190 for centering alignment skirt 170 in casing 100, for ensuring that alignment skirt will mate properly with stinger 130A.
FIG. 1B depicts a top view of casing 100, which also shows the orientation of perforating gun 160 relative to preferred vertical plane 195. Note that perforating gun 160 is loaded with perforating charges 162, positioned at four shots per foot and oriented at a 90.degree. phase from each other. In the above described shot configuration, no perforating charge is oriented in parallel with preferred vertical plane 195. Instead, each of perforating charges 162 is oriented 45.degree. away from the preferred vertical plane. Therefore, it would be expected that the wellbore would produce hydrocarbons at a lower rate than if perforating charges 162 were oriented in parallel with preferred vertical plane 195 . Note that, in the depicted example, only half of the perforating charges can ever be aligned with (or in phase with) preferred vertical plane 195. The remaining charges will be oriented 90.degree. away from preferred vertical plane 195. Therefore, it is expected that the perforations created from those perforating charges will produce hydrocarbons at the lowest possible rate of any orientation.
In an effort to alleviate the above described shortcomings, FIGS. 1C and 1D depict perforating formation 120, using an ultra high shot density perforating gun. FIGS. 1C and 1D show the same basic configuration as depicted in FIGS. 1A and 1B, with the exception of perforating gun 165. Rather than being loaded with four perforating charges per foot at 90.degree. phase displacement, perforating gun 165 is loaded with eight shots per foot at 45.degree. phasing. Note that perforating gun 165 carries twice the number of perforating charges 167 as in the prior example. While loading a perforating gun with high shot density at a relatively low phase displacement increases the chances that formation 120 will be perforated parallel to preferred vertical plane 195, there is no guarantee of success. Furthermore, the number of perforating charges expended in the effort is double, and the expense increases. Finally, because of size constraints of the maximum diameter of the perforating gun due to the interior diameter of the casing wall, at some shot density it is physically impossible to further increase the shot density due to the size of the individual charges. In many cases, smaller perforating charges must be supplemented in order to accommodate the increased shot density. In that case, the penetration of the individual charges is reduced, further reducing the production of hydrocarbons through the perforations.
It would be advantageous to provide a means for ensuring that the perforating charges carried in a perforating gun are oriented in a known direction. It would further be advantageous to orient perforating charges carried in a perforating gun with the preferred vertical plane of a formation, without the expense of using a workover rig. It would be even further advantageous to adapt existing technologies to an orientation means.